Mobile phones, smartphones, etc., are offering not only telephone functions but also more functions as personal digital assistants in recent years. In terms of the size of devices, there is a strong demand for smaller, thinner and lighter devices, which in turn is generating a greater demand for smaller, thinner and lighter components used for such devices. Speakers are facing the same demand, and piezoelectric speakers that utilize the expanding/contracting displacement of piezoelectric elements in 31 directions to provide enhanced displacement amplification based on flexural displacement are used in mobile devices as they can easily be made thinner while ensuring high sound pressures. In addition, piezoelectric speakers are suitable components of mobile devices for which battery life is important, because these voltage-driven speakers consume less power than dynamic speakers.
These piezoelectric speakers are formed by a laminate comprising up to eight layers or so to particularly reduce the driving voltage, which speakers are attached to a metal plate or other shim plate. Here, a piezoelectric speaker constituted by only one laminate piezoelectric body attached to a metal plate is called the unimorph type, while a piezoelectric speaker constituted by laminate piezoelectric bodies polarized in opposite directions, each attached on either side of a metal plate, is called the bimorph type. These unimorph and bimorph piezoelectric speakers are based on the technology described in Patent Literature 1 below, for example. A bimorph piezoelectric speaker may be achieved with only one element, without using a metal plate, by polarizing the top half and bottom half of a laminate piezoelectric element in opposite directions. This one-piece bimorph element offers relatively high efficiency in terms of flexural displacement because it has no extra structure such as a metal plate.